feat: /compress <focus> — guided compression with focus topic (#8017)

Adds an optional focus topic to /compress: `/compress database schema`
guides the summariser to preserve information related to the focus topic
(60-70% of summary budget) while compressing everything else more aggressively.
Inspired by Claude Code's /compact <focus>.

Changes:
- context_compressor.py: focus_topic parameter on _generate_summary() and
  compress(); appends FOCUS TOPIC guidance block to the LLM prompt
- run_agent.py: focus_topic parameter on _compress_context(), passed through
  to the compressor
- cli.py: _manual_compress() extracts focus topic from command string,
  preserves existing manual_compression_feedback integration (no regression)
- gateway/run.py: _handle_compress_command() extracts focus from event args
  and passes through — full gateway parity
- commands.py: args_hint="[focus topic]" on /compress CommandDef

Salvaged from PR #7459 (CLI /compress focus only — /context command deferred).
15 new tests across CLI, compressor, and gateway.

tests/cli/test_compress_focus.py

@@ -0,0 +1,118 @@
+"""Tests for /compress <focus> — guided compression with focus topic.
+
+Inspired by Claude Code's /compact <focus> feature.
+"""
+
+from unittest.mock import MagicMock, patch
+
+from tests.cli.test_cli_init import _make_cli
+
+
+def _make_history() -> list[dict[str, str]]:
+    return [
+        {"role": "user", "content": "one"},
+        {"role": "assistant", "content": "two"},
+        {"role": "user", "content": "three"},
+        {"role": "assistant", "content": "four"},
+    ]
+
+
+def test_focus_topic_extracted_and_passed(capsys):
+    """Focus topic is extracted from the command and passed to _compress_context."""
+    shell = _make_cli()
+    history = _make_history()
+    compressed = [history[0], history[-1]]
+    shell.conversation_history = history
+    shell.agent = MagicMock()
+    shell.agent.compression_enabled = True
+    shell.agent._cached_system_prompt = ""
+    shell.agent._compress_context.return_value = (compressed, "")
+
+    def _estimate(messages):
+        if messages is history:
+            return 100
+        return 50
+
+    with patch("agent.model_metadata.estimate_messages_tokens_rough", side_effect=_estimate):
+        shell._manual_compress("/compress database schema")
+
+    output = capsys.readouterr().out
+    assert 'focus: "database schema"' in output
+
+    # Verify focus_topic was passed through
+    shell.agent._compress_context.assert_called_once()
+    call_kwargs = shell.agent._compress_context.call_args
+    assert call_kwargs.kwargs.get("focus_topic") == "database schema"
+
+
+def test_no_focus_topic_when_bare_command(capsys):
+    """When no focus topic is provided, None is passed."""
+    shell = _make_cli()
+    history = _make_history()
+    shell.conversation_history = history
+    shell.agent = MagicMock()
+    shell.agent.compression_enabled = True
+    shell.agent._cached_system_prompt = ""
+    shell.agent._compress_context.return_value = (list(history), "")
+
+    with patch("agent.model_metadata.estimate_messages_tokens_rough", return_value=100):
+        shell._manual_compress("/compress")
+
+    shell.agent._compress_context.assert_called_once()
+    call_kwargs = shell.agent._compress_context.call_args
+    assert call_kwargs.kwargs.get("focus_topic") is None
+
+
+def test_empty_focus_after_command_treated_as_none(capsys):
+    """Trailing whitespace after /compress does not produce a focus topic."""
+    shell = _make_cli()
+    history = _make_history()
+    shell.conversation_history = history
+    shell.agent = MagicMock()
+    shell.agent.compression_enabled = True
+    shell.agent._cached_system_prompt = ""
+    shell.agent._compress_context.return_value = (list(history), "")
+
+    with patch("agent.model_metadata.estimate_messages_tokens_rough", return_value=100):
+        shell._manual_compress("/compress   ")
+
+    shell.agent._compress_context.assert_called_once()
+    call_kwargs = shell.agent._compress_context.call_args
+    assert call_kwargs.kwargs.get("focus_topic") is None
+
+
+def test_focus_topic_printed_in_compression_banner(capsys):
+    """The focus topic shows in the compression progress banner."""
+    shell = _make_cli()
+    history = _make_history()
+    compressed = [history[0], history[-1]]
+    shell.conversation_history = history
+    shell.agent = MagicMock()
+    shell.agent.compression_enabled = True
+    shell.agent._cached_system_prompt = ""
+    shell.agent._compress_context.return_value = (compressed, "")
+
+    with patch("agent.model_metadata.estimate_messages_tokens_rough", return_value=100):
+        shell._manual_compress("/compress API endpoints")
+
+    output = capsys.readouterr().out
+    assert 'focus: "API endpoints"' in output
+
+
+def test_no_focus_prints_standard_banner(capsys):
+    """Without focus, the standard banner (no focus: line) is printed."""
+    shell = _make_cli()
+    history = _make_history()
+    compressed = [history[0], history[-1]]
+    shell.conversation_history = history
+    shell.agent = MagicMock()
+    shell.agent.compression_enabled = True
+    shell.agent._cached_system_prompt = ""
+    shell.agent._compress_context.return_value = (compressed, "")
+
+    with patch("agent.model_metadata.estimate_messages_tokens_rough", return_value=100):
+        shell._manual_compress("/compress")
+
+    output = capsys.readouterr().out
+    assert "focus:" not in output
+    assert "Compressing" in output